1. Field of the Invention
The present invention relates to the technical field of general communication and, more particularly, to a soft-in-soft-out (SISO) decoder for a general block code.
2. Description of Related Art
Robust and efficient wideband communication is a challenge especially over adverse channels at either single carrier mode or multi-carrier mode of operations. In those communication systems that encounter huge transmission losses and/or require high sensitivity, such as the terrestrial applications that experience severe multipath fading and possible strong interferences, a cascaded forward error correction (FEC) scheme is usually used to provide high decoding gain in order to compensate any possible sources of loss. Toward this end in such scheme, inner decoder usually provides soft information, instead of a hard decision, associated with each coded bit to outer decoder. Those decoders that accept soft information at input and is able to provide outer decoder at with the soft information associated with each coded bit is called a soft-input-soft-output (SISO) decoder. In this invention, a simple and generic SISO decoder for (n, k) block code is proposed.
FIG. 1 is an equivalent block diagram of a general communication system that employs a cascaded coding scheme. At the transmitter site, a source bit sequence is passed through an outer encoder 110. The inner encoder 120 encodes a sequence out of the outer encoder 110. Then, the resulted coded bit stream from inner encoder is mapped to a sequence of QAM symbols by a QAM symbol mapper 125 and finally, the mapped QAM symbol sequence is sent over a communication channel 130.
At the receiving site, assuming perfect synchronization and channel estimation, an inner decoder 140 that acts as counterpart of inner encoder 120 is supposed to decode a block of received QAM symbols out of communication channel 130. In corresponding to the block of received QAM symbols, inner decoder 140 provides the outer decoder with decisions associated with all corresponding source bits encoded by inner encoder 120. The outer decoder 150 receives these decisions and performs a decoding process that acts as counterpart of outer encoder 110 to thereby generate the decoded bit sequence that serves as estimation of the source bit sequence at the input of outer encoder 110.
To achieve high decoding performance, most of commonly used outer codes in communication systems nowadays, such as popular turbo code or low density parity check code (LDPC), require input in the form of soft information. Therefore, it becomes imperative and highly demanded to design a simple and robust SISO inner decoder for a high quality receiver. On performing a soft decision decoding after perfect synchronization is achieved and perfect channel estimation is available, each received QAM symbol can be safely modeled by Gaussian distribution. Owing to the exponential nature of its probability density function (PDF), the soft information in the form of log likelihood ratio associated with each coded bit is commonly used to thereby simplify the computational complexity.
Though design of a SISO decoder for a general block code has been discussed in open literatures and prior arts, they are either complicated or incomplete. Therefore, it is desirable to provide a simple, robust and generic SISO decoder to mitigate and/or obviate the existing problems.